Reducing a 6" take-off collar (supply plenum) to a 4" duct

[aunsafe2015]

A bathroom in my house that needs a max of only about 30 cfm of cooling (~800 btu) is fed by a 6" duct, which is probably supplying about 2x the volume of air that the bathroom actually needs. Consequently, that bathroom is always 3-5 degrees colder than my setpoint.

The 6" duct is connected directly to my A/C's supply plenum. The duct's take-off collar at my plenum has a damper, but closing the damper seems to do nothing. Perhaps the damper is broken.

Because closing the damper is not helping, I was thinking I would replace the 6" duct with a 4" duct. Access to the duct is very easy and I'm guessing it would only be about 30 minutes of work. I was hoping to avoid modifying the supply plenum, so rather than replacing the 6" take-off collar with a 4" take-off collar, I was thinking I would simply attach a 6" to 4" reducer directly to the 6" take-off collar, and then attach my 4" duct to the reducer.

Does anybody know if this should be effective? My only concern is that perhaps the 6" take-off collar will still receive a higher volume of air than I need, and when it hits the 4" reducer the velocity of the air will simply increase so that the total volume remains the same as with the 6" duct.

But I was hoping that since the 4" reducer would be attached directly to the 6" collar, the air flow into the duct would basically be the same as if I had a 4" collar.

Any thoughts appreciated!

 

[yeldogt]

I would fix the damper -- 4" is very small and never seems to be used for supplies.

When I design my houses I always oversize the ductwork a little for the bathrooms used often and master bedroom.

motor damper and thermostat is most likely overkill unless a full bathroom that's occasionally hot or cold. I have done this as well

 

[brewchief]

I would first check the damper, if you do decide to reduce to 4" I would use a cap reducer instead of a taper reducer.

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[Jim greengo]

Can you close the vent part way in the bathroom?

 

[aunsafe2015]

Can you close the vent part way in the bathroom?

Yeah, but then the vent starts making noise...

 

[aunsafe2015]

I would fix the damper -- 4" is very small and never seems to be used for supplies.

When I design my houses I always oversize the ductwork a little for the bathrooms used often and master bedroom.

motor damper and thermostat is most likely overkill unless a full bathroom that's occasionally hot or cold. I have done this as well

Yeah, I suppose I could take the duct off the collar and see what's going on with the damper. Perhaps that's the best idea.

That said, manual J does not lie. A 6 inch duct for a ~100 SQ ft room with only North facing exterior walls and only 1 small window is way more than would ever be needed unless global warming increases the design temp in my area by about 20 degrees [emoji23]

 

[aunsafe2015]

I would first check the damper, if you do decide to reduce to 4" I would use a cap reducer instead of a taper reducer.

Sent from my SM-G965U using Tapatalk

Interesting, are you thinking that a cap reducer is less likely to simply increase velocity than a tapered reducer? I didn't know a cap reducer existed til I just googled it.

 

[mustangacman]

I would get a new collar with damper.
https://www.homedepot.com/p/SPEEDI-...r-HVAC-Duct-Work-Connections-SC-06D/202542804

 

[danski0224]

A bathroom in my house that needs a max of only about 30 cfm of cooling (~800 btu) is fed by a 6" duct, which is probably supplying about 2x the volume of air that the bathroom actually needs. Consequently, that bathroom is always 3-5 degrees colder than my setpoint.

The 6" duct is connected directly to my A/C's supply plenum. The duct's take-off collar at my plenum has a damper, but closing the damper seems to do nothing. Perhaps the damper is broken.

Because closing the damper is not helping, I was thinking I would replace the 6" duct with a 4" duct. Access to the duct is very easy and I'm guessing it would only be about 30 minutes of work. I was hoping to avoid modifying the supply plenum, so rather than replacing the 6" take-off collar with a 4" take-off collar, I was thinking I would simply attach a 6" to 4" reducer directly to the 6" take-off collar, and then attach my 4" duct to the reducer.

Does anybody know if this should be effective? My only concern is that perhaps the 6" take-off collar will still receive a higher volume of air than I need, and when it hits the 4" reducer the velocity of the air will simply increase so that the total volume remains the same as with the 6" duct.

But I was hoping that since the 4" reducer would be attached directly to the 6" collar, the air flow into the duct would basically be the same as if I had a 4" collar.

Any thoughts appreciated!

Typical round residential duct dampers are far from 100% shutoff. Because your duct is connected to the supply plenum, it is also receiving much more air than if it was connected to the ductwork further away.

I would move the connection point further away from the supply plenum, and still run it in 4" ducting, with a damper.

The larger takeoff opening in the supply plenum will act like a funnel, so actually reducing the opening size would help.

If the distance is short, running it in 3" duct might even be an option.